Optical harmonic generation may be used to convert laser light from one wavelength to a shorter wavelength, i.e. a higher frequency. For example, frequency doubling, or second harmonic generation (“SHG”), may be used to obtain visible light from near infrared light. In addition, frequency tripling, also referred to as third harmonic generation (“THG”), may be used to obtain blue, violet, and ultraviolet (UV) light from near infrared light. The frequency doubled and tripled light may then be used for spectroscopy, materials processing, optical pumping, etc.
The optical frequency of laser light may be tripled using cascaded nonlinear optical crystals. Referring to FIG. 1, a prior-art cascaded harmonic tripler 10 is shown as an example. The cascaded harmonic tripler 10 includes sequentially disposed second harmonic 12 and third harmonic 13 crystals, and a dichroic mirror (or filter) 15. In operation, a fundamental light beam 11 at optical frequency ω impinges on the second harmonic crystal 12. Since a nonlinear conversion efficiency is less than 100%, only a portion of the fundamental light beam 11 is frequency doubled, so that a second harmonic beam 14 at a second harmonic frequency 2ω exits the second harmonic crystal 12 together with an unconverted portion 11A of the fundamental optical beam 11. The second harmonic beam 14 and the unconverted portion 11A of the fundamental optical beam 11 impinge on the third harmonic crystal 13, which converts a portion of these beams into a third harmonic beam 19 at a third harmonic frequency 3ω. Thus, three beams exit the third harmonic crystal 13: an unconverted portion 11B of the unconverted portion 11A of the fundamental optical beam 11, an unconverted portion 14A of the second harmonic beam 14, and the third harmonic beam 19. The dichroic mirror 15 redirects the fundamental 11B and second harmonic 14A beam portions, and transmits the third harmonic beam 19 as a desired output.
One drawback of the prior-art cascaded harmonic tripler 10 is that tight focusing of the fundamental 11A and second harmonic 14 beams into the third harmonic crystal 13 is typically required to obtain reasonable conversion efficiency. One drawback of tight focusing is that a small spot diameter of the fundamental 11A and second harmonic 14 beams may compromise beam quality due to a beam walk-off effect. Another drawback is that a UV-induced degradation of the third harmonic crystal 13 output surface may result after tens or hundreds of hours of exposure at UV peak power densities in the 200 MW/cm2 range and average powers in the Watt range or more.